1. Field of the Invention
This invention relates to a method for making a selective absorption film for solar energy which is able to absorb the solar energy efficiently with a reduced loss of once stored heat energy.
2. Description of the Prior Art
The importance of use of solar energy is growing because of the recent, world-wide, difficult energy conditions. In order to effectively utilize the solar energy, there are needed selective solar energy absorption films showing excellent absorption characteristics, i.e. absorption films which exhibit a large absorptivity .alpha. in the wavelength range of visible and near infrared light (0.3-2.mu.) which occupies most of the solar energy and particularly the solar spectrum and which show a small emissivity .epsilon. in the radiation range (2-20 .mu.m) of once stored heat energy.
Known methods of making such selective absorption films of the just-mentioned type include a method of electroplating copper plates with black chromium, black nickel, etc., a chemical coversion method in which oxide films are formed on plates of iron, stainless steel and copper, a method of forming a color-anodized aluminium, a painting or coating method using black paints, and a vacuum deposition and CVD method of interference multi-layer laminates. However, the absorption films obtained by these methods do not always show good selective absorption characteristics and some may present a problem of durability. Further, all of these methods involve some drawbacks from a view point of cost in that treating processes are complicated, that the treating steps are too many in number, and the treating efficiency is low.
On the other hand, it is known to form a black covering or coating on aluminium surfaces by a chemical conversion technique. This technique is simpler than the above-described methods of making selective absorption films. In addition, since such a black coating is able to absorb sunlight in a greater degree, the chemical conversion of aluminium is considered as a simple method for making such a selective absorption method. In this connection, however, known aluminium chemical conversion methods capable of yielding black coatings or films which make use of, for example, potassium permanganate, nickel sulfate, or ammonium molybdate have a disadvantage that the films obtained by these methods exhibit an absorptivity .alpha. of above 0.6 and an undesirably high emissivity .epsilon. of above 0.3 and are thus rather poor in selective absorption characteristics. Accordingly, these methods are not considered useful in making a solar energy absorption film.